Technically every old main characters development is "complete." I mean there's no real other place to take older characters. For example Misty has gotten far stronger as a Gym leader than when she was tagging with Ash, Brock was aimless on the cast for years and now he's a doctor, etc. Other supporting characters like Cilan, Bonnie/Clemont, etc, all did as much as the writers could do with them.
The characters with poor motivation are easier to fit back in the cast, because their reasons for traveling with Ash aren't clearly tied with region-specific goals. That's why Dawn wouldn't work anymore - she's an experienced Coordinator who has already gone through her rookie year of triumphs and disappointments. The same has applied to Iris, with her Axew/Excadrill and Drayden plotlines resolved within Unova. A veteran isn't as interesting to watch as a scrappy rookie, and to reset Dawn would be unnatural...look at Ash.
Gym Leaders don't have to have specific goals to travel because they're already accomplished trainers, but their role in the cast as veterans is to provide personality, commentary, and information.
Given this, I think SM solved many major character questions of prior series by having a stationary setting. Now the cast could afford to have smaller-scale goals or no goals at all. They could show more consistent, believable interests and habits instead of relying on running jokes or being a connoisseur of everything like Cilian was.
Ironically...I think had Serena not kissed Ash, her development was poor enough, and motivation nebulous enough, she could have slotted back in fairly easily for a future series. But that kiss was a permanent step forward and she can never get a second chance at redemption.